1. Field of the Invention
The present invention relates to a receiving circuit for use in a transmission system of a digital signal with a transmission baud rate, and in particular, to such a receiving circuit of a baud rate sampling type wherein the digital signal is sampled as a received signal at a sampling rate equal to the transmission baud rate.
2. Description of the Prior Art
In the prior art, a transmitter and a receiver in the digital transmission system have been composed of analog circuits. However, according to recent progress of a switched capacitor filtering (SCF) technique and a digital signal processing technique, it has already been possible to treat and process a series of random signals received at a receiver discretely on a time axis.
Such a receiver comprises a baud rate sampler for sampling the received signal at a sampling rate equal to the transmission baud rate to produce a series of sampled signals and a decision circuit for deciding the series of the sampled signals. The sampling manner using the sampling rate equal to the transmission baud rate is called a baud rate sampling. Another sampling manner is also possible wherein the received signal is sampled at a sampling rate twice or more of the transmission baud rate. However, the former enables a signal processing time and a filter tap number to be reduced in, for example, a line equalizer and is therefore superior to the latter.
A difficulty from which the baud rate sampling suffers is control of the sampling phase, that is, timing control for sampling.
As a technique for resolving the difficulty, Muller et al proposed to utilize an impulse response of the series of received signals and a decided result at the decision circuit in their paper "Timing Recovery in Digital Synchronous Data Receiver", IEEE Transaction on communication, VOL. COM-24, NO. 5, MAY 1976 (Reference 1). However, this technique has a problem that correct operation is not expected when the decided result includes many errors, for example, at a training phase of the receiver operation.
Sailer et al proposed another technique to control the sampling phase by use of a special code which is called "Barker Code" in their paper "A VLSI Transceiver for the ISDN Customer Access", Globecom. 1985 (Reference 2). However, it is a problem per se to use the special code.